Allocating data between tones

ABSTRACT

A new protocol is proposed for transmission of data through lines such as telephone lines. The tones of a signal are associated into groups  10, 11 , and tone ordering, gain selection, and/or bit swapping within the processing system are done within the members of a group  10, 11 . This idea is applicable both to tone ordering etc., following the training stage, and also to the dynamic configuration changes subsequently, for example, bit swapping. The computational cost of coding and decoding the data is reduced (compared to treating all the tones of a given direction equivalently), and the invention makes it possible to significantly reduce the memory requirements of the encoder and decoder.

This application is a continuation of patent application Ser. No.11/073,000, entitled “Allocating Data Between Tones in a VDSL System,”filed on Mar. 4, 2005 now U.S. Pat. No. 7,496,144 and which claimspriority to Singapore Patent Application 200401383-5, filed on Mar. 5,2004, both of which are incorporated herein by reference.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application relates to the following co-pending and commonlyassigned patent applications: Ser. No. 11/072,992, entitled“Computationally Efficient Protocols for VDSL System”, Ser. No.11/071,987, entitled “VDSL Protocol with Low Power Mode”, and Ser. No.11/073,001, entitled “Trellis Modulation Protocols for a VDSL System”,which applications are hereby incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to methods for transmitting data, inparticular over telephone lines (typically, copper telephone lines) orsimilar lines. It further relates to systems arranged to perform themethods.

BACKGROUND

The use of fast Internet connections has grown rapidly over the last fewyears, and consequently the demand for broadband (high-speed)connections is increasing.

One technology that is very well known in the market is AsymmetricDigital Subscriber Line (ADSL) technology. This employs the frequencyspectrum indicated schematically in FIG. 1. “Upstream” communications(that is in the direction from the home or office user premises,“customer premises equipment” or “CPE”, to the “central office”, or “CO”or DSLAM, FTTC, or Fibre To The Curb, or FTTH, Fibre To The Homecabinets) are transmitted on frequencies in the range of 25 kHz (i.e.,above the maximum audible frequency of 4 kHz) to 138 kHz. “Downstream”communications are in a higher frequency band from 138 kHz to an upperlimit. According to the first two versions of ADSL (ADSL and ADSL2) thedownstream band goes up to 1.1 MHz, whereas in ADSL2+ it goes up to 2.2MHz. The upstream can be also extended from 0 kHz up to 276 kHz, alsoknown as All Digital Loop and extended upstream. Within each of theupstream and downstream bands, the range is divided into 4 kHzintervals, “tones,” so that the downstream band includes 256 tones inADSL and ADSL2 (which is capable of transmitting 8 Mbps), and 512 tonesin ADSL2+ (which is capable of transmitting 28 Mbps). Each tone isencoded by quadrature amplitude modulation (“QAM”), and can encodebetween 0 and 15 bits. During a training phase, the line conditions(signal to noise ratio, SNR) of each of the tones is estimated, and thenumber of bits which will be encoded in each tone during each frame isselected.

In a typical ADSL modem, the main sections are (i) a Digital Interface(which may use asynchronous transfer mode (ATM)); (ii) a Framer (alsoreferred to here as a framing unit); (iii) a Discrete MultiTone (DMT)Modulator; (iv) the AFE (Analog Front End); and (v) a Line Driver.

The framer multiplexes serial data into frames, generates FEC (forwarderror correction), and interleaves data. FEC and data interleavingcorrects for burst errors. This allows DMT-based ADSL technology to besuitable for support of MPEG-2 and other digital video compressiontechniques. For the transmit signal, an Encoder encodes frames toproduce the constellation data for the DMT Modulator. It assigns themaximum number of bits per tone (based on measured SNR of each tone) andgenerates a QAM constellation where each point represents a digitalvalue. Each constellation point is one of N complex numbers, x+iy, wherex and y are the phase and amplitude components. The summation of bits inall carriers, multiplied by the frame rate (4 kHz), represents the datarate. For the receive signal, the decoder converts QAM symbols back intothe data bitstream.

In the DMT Modulator, a frequency domain processor implements FFT/IFFTand associated processing. In the transmit path, the Inverse FastFourier Transform (IFFT) module accepts input as a vector of N QAMconstellation points and duplicates each carrier with its conjugatecounterpart so the 2N output samples are real. The 2N time domainsamples may have for example the last 2N/16 samples appended as a cyclicextension (which may include a cyclic suffix, a windowing functionand/or a cyclic prefix extension) for every symbol, and are thendelivered to a DAC (digital-to-analog converter). The set of time domainsamples represents a summation of all the modulated sub-channels, forthe duration of one data frame. In the receive path, the first 2N/16samples (cyclic prefix) from the ADC are removed from every symbol. AFFT module transforms the carriers back to phase and amplitudeinformation (N complex QAM symbols). Correction for attenuation of thesignal amplitude and phase shifts (i.e., overall distortion) isimplemented. If the QAM constellation is thought of as points in a gridwhere rows and columns represent phase and amplitude informationrespectively, then the grid effectively rotates reference to theconstellation points to correct for these distortions.

Based on the SNR, which has been established for the tones, they areclassified based on the SNR such that a “path” is selected for each tonethrough the encoding device, and each of the tones is transmitted alongto the framing unit through the corresponding selected transmissionpath. This is illustrated in FIG. 2( a), in which the framing unit 1 forproducing V/ADSL frames receives data along two paths 2, 3. Each path 2,3 leads to a respective block 4, 5, which constructs respective portionsof frames. The frame is shown in FIG. 2( b), including a portion 6generated by block 8, and a portion 7 constructed by a block 9 (whichmay be an interleaver). The outputs of the blocks 4, 5 are storedrespectively in a fast buffer 8 and interleaved buffer 9, until they aretransmitted out of the framing unit 1. Since the interleaver 5interleaves data over a period of time, data transmitted along path 3will have a different (higher) latency than data transmitted along thepath 2. Thus, these two paths are referred to as different “latencypaths” (e.g., they may be referred to as LP1 and LP2). Note that bothpaths LP1 and LP2 may be interleaved.

DMT technology also includes a feature known as “tone ordering.” Thismeans that the encoder, in forming VDSL symbols (there may be multipleVDSL frames within one VDSL symbol), determines the order in whichsubcarriers are assigned bits. The term tone ordering is wide enough toinclude both (i) determining the order in which the subcarriers areassigned data transmitted along a given latency path; and (ii) the orderin which the subcarriers are assigned data transmitted along thedifferent latency paths.

Furthermore, the number of bits that are transmitted by each of thetones may be modified if the estimated SNRs of the tones are revised:increasing the number of bits stored per frame in some tones andcorrespondingly reducing the number of bits stored per frame in othertones. There could be other reasons to dynamically change the bitallocation for spectral reasons too. This process is known as “bitswapping.”

For further details of the ADSL2 standard, the reader is referred to thedocument ITU-T Recommendation G.992.3 published by the InternationalTelecommunication Union, the disclosure of which is incorporated hereinby reference in its entirety.

While ADSL provides Internet connections that are many times faster thana 56 k modern, they still are not fast enough to support the integrationof home services such as digital television and Video-on-Demand.However, another DSL technology known as very high bit-rate DSL (VDSL)is seen by many as the next step in providing a completehome-communications/entertainment package.

In contrast to ADSL, a conventional VDSL standard (here referred to asVDSL1) uses a number of bands, e.g., as shown in FIG. 3, which may go upto, for example, 12 MHz. Data rates are typically larger than those ofADSL, e.g., 8 k samples per VDSL symbol for 4096 point-FFT. VDSL has anumber of further differences from ADSL. For example, VDSL1 hasdifferent framing methods from ADSL2 (for example, with no sync symbol),it does not include Trellis encoding, and its interleaving system isdifferent. In the ADSL2 system, the tone ordering is applied to all thetones used for communication in a given direction. Up until now, twosets of memories were required on a chip. If this feature isincorporated into future versions of VDSL, here referred to as VDSL2,with 4 k tones or higher, each of the bit allocation table, gain tables,tone ordering tables each for 4 k tones requires significant on-chipmemory.

SUMMARY OF THE INVENTION

Embodiments of the present invention aim to provide new and usefulprotocols for transmitting data through lines such as telephone lines.Typically these protocols have transmission rates of over 24 Mbps, andoften much higher.

Embodiments of the present invention propose in general terms that thetones of a signal are grouped, a portion of data is associated with eachof the groups of tones, and that the tones of the group are usedtogether to transmit the corresponding portion of data (e.g., in a waydetermined by a measured SNR of each tone, or possibly by a selectionmade by the unit receiving the transmitted data).

A selection may be made from the tones of the group such that each ofthe tones of the group is allotted a respective role in transmitting thedata.

Thus, any one or more of tone ordering, bit swapping, and/or selectionof gains for each tone (e.g. gain scaling) is done within the members ofa group.

This idea is applicable both to tone ordering etc., following thetraining stage, and also to the dynamic configuration changessubsequently, for example, bit swapping.

Implementations of the invention may offer the advantage (compared totreating all the tones of a given direction equivalently for thepurposes of bit allocation, bit swapping, gain allocation and toneordering) that the computational cost of coding and decoding the data isreduced.

Furthermore, embodiments of the invention make it possible tosignificantly reduce the memory requirements of the encoder and decoder.By contrast, the current tone re-ordering system implemented in an ADSLsystem allows for tone re-ordering to happen across all the tones duringthe seamless rate adaptation. To allow that, it is required to have twocopies of the constellation gains, bit allocation tables and tonere-ordering tables for all the tones. For an VDSL2 system with potential4 k tones, this requires significant memory on the chip. By means ofapplying tone ordering etc., only to the tones of groups, it is proposedto reduce the above requirements and provide provision to achieve thepossible means to dynamically adapt the data rates.

The groups may be defined in various ways within the scope of theinvention, and need not be the same.

A first possibility is for the groups to correspond to the bands (i.e.,that all the consecutive tones that transmit information in a givendirection are in the same group).

A second related possibility is for there to be multiple groups withineach of the bands, but such that all the tones of any given group areall within a single one of the bands.

A third possibility is for the groups to be selected irrespective of thebands, e.g., such that each group of tones are consecutive ones of thetones for transmitting data in a given direction. For example, thegroups of upstream (downstream) tones may be consecutive, with some ofthe groups extending over more than one of the upstream (downstream)bands.

In any of these three possibilities, the groups may be predefined, i.e.,as part of the protocol. However, this feature is not required by theinvention, which may instead allow the groups to be defined based on theSNR ratios.

For example, a fourth possibility is for the groups to be defined basedon the SNR ratios, but for the number of tones in each group to belimited. In one form of this possibility, for example, the tones of agiven group may be selected from any of the tones associated with agiven direction of communication (e.g., based on conventional toneordering) but each group is limited to a maximum number of tones. Thistoo would help to ensure that errors are not propagated between thegroups.

These various possibilities can be performed in combination. Forexample, a further way to select groups would be based on SNR ratios (asin the fourth possibility) but such that the tones of any given groupare within only a single one of the bands (as in the first possibility).

A particularly preferred example is for tone ordering etc., to beperformed during the learning phase for tones that are members of groupsthat are entirely within a single band. Furthermore, the latency pathsetc., selected for each tone are preferably determined based on the SNRsonly of that tone and the other tones that are members of the samegroup. Subsequently, bit swapping is only performed to re-allocate thebits between the tones of these same groups.

In one of the related patent applications referenced above, it isproposed that the tones of a signal are grouped, and that Trellisencoding is performed on the data which is to be transmitted on thetones of each group. This concept is freely combinable with the presentinvention. One possibility is for the same groups, which are used forTrellis encoding, to be used to perform any one or more of bitallocation, bit swapping, selection of gains for each tone and/orselection of paths for each tone. Alternatively, different groups can beused for the Trellis encoding from those that are used for any one ormore of the other four schemes.

In another of the related patent applications referenced above, wepropose that the transmission protocol includes at least two modesdefined by respective band plans. Multiple modes may be useful when thevolume of data that is to be transmitted is reduced, e.g. to save powerrequirements for encoding and transmission and/or reception anddecoding. In one form of this idea, at least a first band planassociated with a first of these modes employs a maximum frequency fordata transmission, which is higher than the maximum frequency used fordata transmission in a second band plan associated with a second of themodes.

Embodiments of the present invention are advantageously combinable withthis idea, since in the case that the groups are defined (e.g.,predefined) to include groups of low frequency and groups of relativelyhigh frequency, the transition from one protocol to the other can beperformed by simply ceasing to transmit data on a plurality of the highfrequency groups of tones. (Note that this does not necessarily implythat nothing at all is transmitted on these high frequencies. Rather,there may still be a transmission of signals, but these signals do notinclude encoded data. They may for example be predefined, or essentiallyrandom. One reason for continuing to broadcast signals at thesefrequencies may be that ceasing to do so would itself change the noiseenvironment.)

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred features of the invention will now be described, for the sakeof illustration only, with reference to the following figures in which:

FIG. 1 shows the frequency usage of a conventional ADSL technique;

FIG. 2, which is composed of FIGS. 2( a) and 2(b), illustrates toneordering in a conventional ADSL technique;

FIG. 3 shows the frequency usage of a conventional VDSL technique;

FIG. 4 shows schematically grouping of tones in a first embodiment ofthe invention;

FIG. 5, which is composed of FIGS. 5( a) to 5(d), shows two specificexamples of grouping the tones in embodiments of the invention;

FIG. 6, which is composed of FIGS. 6( a) and 6(b), shows two possiblestructures of data transmission apparatus according to the embodiment;

FIG. 7, which is composed of FIGS. 7( a) and 7(b), shows schematicallythe variation of data transmission rate in certain embodiments of theinvention; and

FIG. 8, which is composed of FIGS. 8( a) to 8(f), shows possible bandplans in accordance with FIG. 7.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Referring to FIG. 4, a schematic view is shown of bandwidth allocationin a protocol, which is a first embodiment of the invention. Manyfeatures of the protocol, which are not described, are generallyaccording to the ADSL standard (which is described for example in thedocument ITU-T Recommendation G.992.3 published by the InternationalTelecommunication Union), which standard is incorporated herein byreference. In particular the protocol preferably includes any one ormore of the following: (i) tone ordering (for determining the order inwhich subcarriers are assigned bits); (ii) bit allocation (i.e.,selection of the number of bits to be transmitted on each tone); (iii)gain selection; and (iv) bit swapping, all carried out on agroup-by-group basis. Optionally also, Trellis encoding may be performedbased on the same groups.

Generally speaking, in this first embodiment the upstream and downstreamallocation of the bandwidth may be as in VDSL (i.e. as shown in FIG. 3),but within the scope of the invention any other allocation is alsopossible. Indeed, an embodiment of the invention having a novelallocation of the bandwidth into bands is described in one of therelated applications referenced above.

The first embodiment has the feature that in at least one, and morepreferably both, of the upstream and downstream directions, the tonesare grouped into groups 10, 11 (in FIG. 4 the downstream groups arelabeled 10 and the upstream groups are labeled 11). In various versionsof the first embodiment within the scope of the invention this groupingis done in various ways. A first possibility, for example, is topredetermine the number of tones in each group 10, 11 (e.g., such thatall groups are the same number N of tones, such as 4 tones), and then toallocate tones to groups according to a simple scheme, for example suchthat the groups 10 are respective sets of N consecutive tones.

In the first embodiment of the invention, the groups are used forencoding the data. The operations of selecting the number of bits to betransmitted per tone and/or selection of the gains and/or selection oflatency paths and/or bit swapping, are performed only in relation to thetones of single groups 10, 11. For example, within a certain group theSNR of each of the tones may be established during a training phase, andbased on this SNR value a selection may be made of which of a pluralityof paths each of the tones in the groups should be transmitted along.The plurality of paths preferably include at least one path withinterleaving, and at least one path without interleaving. The two pathsmay, for example, be generated by a system as shown in FIG. 2( a).However, note that the invention is not limited in this respect, and inparticular there may be more than two possible paths.

FIG. 5 shows the downstream and upstream spectrum for VDSL1 servicesaccording to two more specific forms of this embodiment of theinvention. All three forms share the bands illustrated in FIG. 5( a):two upstream bands are shown as US1 and US2, and two downstream bandsshown as DS1 and DS2. As shown in FIG. 5( b) and FIG. 5( c), a firstform of the embodiment proposes that each of the downstream and upstreamfrequency bands respectively be further grouped in several groups GD1, .. . GDn and, GU1, . . . GUm where n and in are respectively the numberof groups for downstream and upstream transmission. It will be notedthat GD3 is shown including tones from both DS1 and DS2, while GU3 isshown including tones from both US1 and US2. However, in other forms ofthe invention the groups may be defined such that each group is entirelywithin one of the bands (i.e., a given band may be partitioned intogroups, e.g., such that each of the groups is a set of consecutivetones).

Alternatively, as shown in FIG. 5( d) (which shows, for the sake ofexample only, the downstream direction), the frequency tones within eachof the groups may also span across the complete frequency spectrum (i.e.the groups include members from both bands of the downstream direction),which could be more efficient for Trellis encoding and could be alsodependent on the latency paths or tone ordering. In FIG. 5( d) areaswith the same shading level represent tones within the same group. Thus,for example, the tones of FIG. 5( d) labeled GD1 are the tones thattogether constitute the band GD1.

Note that certain embodiments of the invention include Trellis encoding;others do not. Let us consider for a moment embodiments that do includeTrellis encoding. In both forms of the invention (i.e. the one shown byFIGS. 5( b) and 5(c), and the one shown by FIG. 5( d)), the set of tonesencoded together by Trellis encoding may be defined completelyindependently of the groups described above. For example, all tonesassociated with a given direction may be Trellis encoded together. It isalso possible that the frequency band DS1 be covered with a singleTrellis group and the frequency band DS2 to be covered with secondsingle Trellis group, and similarly for the upstream bands. That is, thegroups used for Trellis encoding may have nothing to do with the groupsused for bit allocation, bit swapping, tone ordering and/or gainselection.

Alternatively, the same groups illustrated in FIGS. 5( b) and 5(c) or inFIG. 5( d) can be used for Trellis encoding. That is, from the point ofview of FIG. 4, Trellis encoding may be used only to combine the N toneswithin the groups 10, 11. The decoding of such data is much cheapercomputationally than if Trellis encoding is performed encoding togetherdata that is to be placed in all the tones (as in ADSL). Thus, thecomputational cost of decoding the data is much reduced. For example, ifthe Trellis encoder employs four states in its algorithm, then decodingof the Trellis data can be performed in about 20 stages in a Viterbidecoder (that is, the number of states multiplied by a parameter, whichfrom statistical theory is known to be about 5). If the number of tonesin each group is lower, then the computational tasks to be performedcould be made more efficient and simpler.

FIG. 6( a) shows the structure of a communication apparatus for use ingenerating the protocol as described above with reference to FIG. 4 orFIG. 5. It includes a framer unit 61, which receives data to betransmitted and arranges it into frames, an interleaver 62, a QAMencoder 63 (which is where the Trellis modulation, bit allocation andtone ordering occur). Note that the data taken into the encoder is takenfrom both latency paths LP0 and LP1, and the control unit of the encoder63 of the tone ordering. The apparatus also includes an IFFT unit 64, afilter 65, a digital-to-analog converter 66 and a line driver 67. Thisstructure is not new, although the operation of the encoder 63 (andoptionally the interleaver 62) is different from known systems toproduce the protocols of the invention. The interleaver 62 includesvarious paths with different latencies, and tone ordering consists ofallocating data to different ones of the tones.

The communication apparatus generally includes other elements, such as aquality determination unit for determining the SNR of each of the toneson the line (this unit may in principle be located outside thetransmitter, such as at the other end of the telephone line, andarranged to transmit its results to the transmitter). Additionally,there is a processor for controlling the operation of one or more of theunits shown in FIG. 6( a) (especially the encoder 63) on the basis ofthe output of the quality determination unit.

FIG. 6( b) shows the structure of a second transmission apparatus foruse in generating the protocol as described above. In contrast to thestructure of FIG. 6( a), in FIG. 6( b) there is an additional secondinterleaver 68 located after the QAM encoder 63. The purpose of thesecond interleaver 68 is to simplify the required operation of the firstinterleaver 62 and the encoder 63 in case the grouping is a particularlycomplex one, such as the coding of FIG. 5( d), in which the tones of agiven group are not at consecutive frequencies. The protocol couldincrease the performance of the system.

Note that the interleaver(s) 62, 68 may be any of a convolutionalinterleaver, a triangular interleaver or a general convolutionalinterleaver (these terms are well-defined in this field).

We now describe the ways in which tone ordering, bit allocation, gainallocation and bit-swapping are performed in the first embodiment of theinvention as described above in relation to FIG. 4 and/or FIG. 5. Thisis performed using many of the techniques known already for toneordering etc., and reference is made in particular to the ADSL2 standardG992.3, and in particular FIGS. 7 and 8.

Specifically, during initialization, the receive PMD function shallcalculate the numbers of bits and the relative gains to be used forevery subcarrier, as well as the order in which subcarriers are assignedbits (i.e., the tone ordering). The calculated bits and gains and thetone ordering shall be sent back to the transmit PMD function during alater stage of initialization (see 8.5.3.3 of the standard).

The pairs of bits and relative gains are defined, in ascending order offrequency or subcarrier index i, as a bit allocation table b and gaintable g (i.e., bi and gi, for i=1 to NSC−1, with b1 bits to be allocatedto subcarrier 1 and bNSC−1 bits to be allocated to subcarrier NSC−1). IfTrellis coding is used, the receive PMD function shall include an evennumber of 1-bit subcarriers in the bit allocation table b.

The tone ordering table t is defined as the sequence in whichsubcarriers are assigned bits from the input bitstream (i.e., ti for i=1to NSC−1, with constellation mapping beginning on subcarrier t1 andending on subcarrier tNSC−1). The tone ordering table t shall remainstatic for the duration of the session.

Following receipt of the tables b, g and t, the transmit PMD functionshall calculate a reordered bit table b′ and a reordered tone table t′from the original tables b and t. Constellation mapping shall occur insequence according to the re-ordered tone table t′, with the number ofbits per tone as defined by the original bit table b. Trellis codingshall occur according to the reordered bit table b′.

If Trellis coding is not used, b′=b and t′=t.

If Trellis coding is used, the re-ordering of table t shall be performedby the transmit PMD function. The re-ordered tone table t′ shall begenerated according to the following rules:

-   -   Indices of all subcarriers supporting 0 bits or 2 or more bits        appear first in t′, in the same order as in table t.    -   Indices of all subcarriers supporting 1 bit appear last in table        t′, in the same order as in table t.

If the bit allocation does not include any 1-bit subcarriers, there-ordered tone table t′ is identical to the original tone table t.

The (even number of) 1-bit subcarriers shall be paired to form2-dimensional constellation points as input to the Trellis encoder. Thepairing shall be determined by the order in which the 1-bit subcarriersappear in the original tone ordering table t.

The table b′ is generated by scanning the re-ordered tone table t′ andre-ordering the entries of table b according to the following rules(with NCONEBIT representing the number of 1-bit subcarriers in the bitallocation table b):

-   -   The first NCONEBIT/2 entries of b′ shall be 0, where NCONEBIT is        the (by definition, even) number of subcarriers supporting 1        bit.    -   The next entries of b′ shall be 0, corresponding to the        subcarriers that support 0 bits.    -   The next entries of b′ shall be non-zero, corresponding to the        subcarriers that support 2 or more bits. The entries shall be        determined using the new tone table t′ in conjunction with the        original bit table b.    -   The last NCONEBIT/2 entries of b′ correspond to the paired 1-bit        constellations (i.e., 2 bits per entry).

The table b′ is compatible with the G.992. 1 Trellis encoder.

The tables b′ and t′ shall be calculated from the original tables b andt as shown in the tone pairing and bit re-ordering processes below.

/* TONE RE-ORDERING PROCESS */ t_index=1; /* tone order index t_index isindex of array t */ t′_index=1; /* tone paired index t′_index is indexof array t′ */ while (t_index<NSC) { tone=t[t_index++]; bits=b[tone]; if(bits==0) { t′[t′_index++]=tone; } if (bits==1) { } if (bits≧2) {t′[t′_index++]=tone; } } while (t′_index<NSC) t′[t′_index++]=1; /* BITRE-ORDERING PROCESS */ NC1=0; /* NCONEBIT is the number of tones with 1bit */ NCL=0; /* NCUSED is the number of used tones (at least 1 bit) */for (i=1; i<NSC; i++) { if (b[i]>0) NCL++; if (b[i]==1) NC1++; }b′_index=1; while (b′_index<(NSC−(NCUSED−NCONEBIT/2))) b′[b′_index]=0;t′_index=1; while (t′_index<NSC) { tone=t′[t′_index++]; bits=b[tone]; if(bits==0) { } if (bits==1) { b′[b′_index++]=2; t′_index++; } if (bits≧2){ b′[b′_index++]=bits; }

If on-line reconfiguration changes the number or indices of 0-bitsubcarriers or 1-bit subcarriers, then tables t′ and b′ shall berecalculated from the updated table b and the original table t.

The constellation encoder takes L bits per symbol from the PMS-TC layer.If Trellis coding is used, the L bits shall be encoded into a number ofbits L′ matching the bit allocation table b and the re-ordered bit tableb′, i.e., into a number of bits equal to L′=Σb_(i)′=Σb_(i). See 8.6.2 ofthe standard. The value of L and L′ relate as:

$L^{\prime} = {{\sum b_{i}^{\prime}} = {{\sum b_{i}} = {L + \left\lceil \frac{{NCUSED} - \frac{NCONEBIT}{2}}{2} \right\rceil + 4}}}$

with the ┌x┐ notation representing rounding to the higher integer. Theabove relationship shows that using the 1-bit subcarrier pairing method,on average, one Trellis overhead bit is added per set of four 1-bitsubcarriers, i.e., one Trellis overhead bit per 4-dimensionalconstellation. In case Trellis coding is not used, the value of L shallmatch the bit allocation table, i.e., L=Σb_(i).

A complementary procedure should be performed in the receive PMDfunction. It is not necessary, however, to send the re-ordered bit tableb′ and the re-ordered tone table t′ to the receive PMD function becausethey are generated in a deterministic way from the bit allocation tableand tone ordering tables originally generated in the receive PMDfunction, and therefore the receive PMD function has all the informationnecessary to perform the constellation demapping and Trellis decoding(if used). Alternatively, it means that the receive PMD could only sendthe delta information of the re-ordered table and need not send thecomplete table. In a further version, it may not be required to maintainthe original table but only work with delta tables for any laterchanges.

All these features are incorporated in the first embodiment. If, asdiscussed above, the groups of the Trellis coding are as shown in FIGS.5( b) and 5(c), or alternatively in FIG. 5( d), the tone ordering etc.,may be performed using only the tones of the same groups used forTrellis encoding, e.g., with all the tones of the group transmittedusing the same latency path.

For the online reconfiguration, for efficient implementation, one optionis to limit the tone ordering process within a group for a singlelatency path. The bit-swap procedure should be limited to one group atone time. After the procedure is completed for one group, the bitswapping procedure is extended to the next group.

Some of the related patent applications referenced above describesystems in which the protocols permit a low power mode of operation,principally saving power consumption in the IFFT in the case of theencoder, or the FFT in the case of the decoder. This is illustrated inFIG. 7( a), which shows switching at different times between two modesL₁ and L₂. In these two modes the data transmission rate may bedifferent.

In fact, there may be a choice of different power saving modes. This isillustrated in FIG. 7( b) in which the protocol uses a first powersaving mode L₂ at a first time, and a second power saving mode L₄ atother times.

FIG. 8 shows various specific band plans which can be produced in thevarious modes of operation explained in these related applications. FIG.8( a) shows the full frequency range (up to 17 MHz) being used fortransmitting data (i.e., the VDSL high power mode). (Otherwisefrequencies up to 30 MHz may be used).

FIG. 8( b) shows a different mode of operation, in which onlyfrequencies up to 1.1 MHz are used for transmitting data. Thosefrequencies above 1.1 MHz are shown with low values to indicate that nosignal is transmitted on those frequencies, or (in other forms of theembodiments) that signals are transmitted which do not carry data. Forexample, it may be advantageous to broadcast signals which do not carrydata on frequencies 1.1 MHz to 17 MHz, to avoid changing the noiseenvironment.

In yet further versions, the maximum transmission rate may be lower forthe range of frequencies, which are shown as having a low value. Inother words, the range of frequencies (a “high frequency range”) between1.1 MHz and 17 MHz may be used to carry a certain data load in the firstmode of operation, and a lower amount of data in the mode of FIG. 8( b).

FIG. 8( c) shows a third mode of operation in which frequencies up to 12MHz are used for transmitting data. Just as described above in relationto FIG. 8( a), the frequencies above 12 MHz which are shown as “low” maybe unused, used for non-data transmission, or used for data transmissionat a lower data transmission rate than in FIG. 8( a).

FIG. 8( d) shows a fourth mode of operation in which frequencies up to 8MHz are used for transmitting data. Just as described above in relationto FIG. 8( a), the frequencies above 8 MHz which are shown as “low” maybe unused, used for non-data transmission, or used for data transmissionat a lower data transmission rate than in FIG. 8( a).

FIG. 8( e) shows a fifth mode of operation in which frequencies up to5.3 MHz are used for transmitting data. Just as described above inrelation to FIG. 8( a), the frequencies above 5.3 MHz which are shown as“low” may be unused, used for non-data transmission, or used for datatransmission at a lower data transmission rate than in FIG. 8( a).

FIG. 8( f) shows a sixth mode of operation in which frequencies up to4.4 MHz are used for transmitting data. Just as described above inrelation to FIG. 8( a), the frequencies above 4.4 MHz which are shown as“low” may be unused, used for non-data transmission, or used for datatransmission at a lower data transmission rate than in FIG. 8( a).

These concepts are freely combinable with embodiments of the presentinvention. For example, in the case that the groups of the embodimentare defined (e.g., predefined) to include groups of low frequency andgroups of relatively high frequency (i.e., all the tones of ahigh-frequency group are of higher frequency than all the tones of a lowfrequency group), the transition from one protocol to the other can beperformed by ceasing to transmit data on one or more of the highfrequency groups of tones (either ceasing to transmit a signal at allthe frequencies contained in the high-frequency groups, or continuing totransmit signals at those frequencies, but arranging that those signalsdo not include data). This combination is particularly suitable in thecase that no groups include tones from more than one band.

During the low power modes the transmission may be restricted to only afew bands. Hence, if it is allowed to smoothly transit from one mode oftransmission to another, it allows for smooth transition of the mode. Inlow power mode, the same set of tone ordering tables and bit gain tablescould be used with the exception that the bands not used fortransmission during the low power modes may not be used. The Trellispairs also will not be affected and hence allows for seamless transitionto low power mode. This mode of operation provides efficientimplementation.

Although only a few embodiments of the invention have been disclosed inthis application, many variations are possible within the scope of theinvention as will be clear to a skilled reader. For example, possibleembodiments of the invention exist wherein the modulation technique isother than one using an IFFT.

What is claimed is:
 1. A method comprising: separating a plurality oftones for one user into a plurality of groups of tones within a band ofonly one direction of a bi-directional data communication; modulatingeach tone of the plurality of tones in accordance with a respective bitallocation for that tone; and modifying the respective bit allocationfor tones of the plurality of tones on a group-by-group basis within afirst group of the plurality of groups.
 2. The method according to claim1, wherein modifying the respective bit allocation further comprises:after the modifying of the respective bit allocation of tones within thefirst group is completed, modifying the bit allocation of tones on agroup-by-group basis within a second group of the plurality of groups.3. The method according to claim 1, wherein the modifying of therespective bit allocation is a bit swapping procedure during an onlinereconfiguration.
 4. The method according to claim 1, wherein all thetones of at least one group are within a single band of thebi-directional data communication.
 5. The method according to claim 1,wherein there are multiple groups within a single band of thebi-directional data communication.
 6. The method according to claim 1,wherein one or more of the groups includes tones within at least twodifferent respective bands of the bi-directional data communication. 7.The method according to claim 1, wherein a single band of the bidirectional data communication is between a single user and a centraloffice.
 8. The method of claim 1, wherein a computational cost of codingand decoding the tones when separating the plurality of tones is lowerthan a computational cost of coding and decoding the tones when notseparating the plurality of tones.
 9. The method of claim 8, wherein thecomputational cost comprises a memory requirement of coding and decodingthe tones.
 10. A device comprising: a tone manager configured toseparate a plurality of tones for one user into a plurality of groups oftones within a band of only one direction of a two directional datacommunication; and a modulator configured to modulate each tone of theplurality of tones according to a respective bit allocation for thattone, wherein the device is configured to modify the respective bitallocation on a group-by-group basis of tones within a first group ofthe plurality of groups.
 11. The device according to claim 10, whereinthe modulator is further configured to modify the respective bitallocation on a group-by-group basis of tones within a second group ofthe plurality of groups tones after the modifying of the respective bitallocation of the tones within the first group is completed.
 12. Thedevice according to claim 10, wherein the device is configured to modifythe respective bit allocation in a bit swapping procedure during anonline reconfiguration.
 13. The device according to claim 10, whereinall the tones of at least one group are within a single band of the twodirectional data communication.
 14. The device according to claim 10,wherein there are multiple groups within a single band of the twodirectional data communication.
 15. The device according to claim 10,wherein one or more of the groups includes tones within at least twodifferent respective bands.
 16. The device according to claim 10,wherein a single band of the bi directional data communication isbetween a single user and a central office.
 17. A chip comprising: afirst entity arranged to separate a plurality of tones for one user intoa plurality of groups of tones within a band of only one direction of atwo directional data communication; and a second entity arranged tomodulate each tone of the plurality of tones according to a respectivebit allocation for that tone, wherein the chip is configured to modifythe respective bit allocation on a group-by-group basis of tones withina first group of the plurality of groups.
 18. The chip according toclaim 17, wherein the second entity is further configured to modify therespective bit allocation on a group-by-group basis for tones within asecond group of the plurality of groups after the modifying of therespective bit allocation of tones within the first group is completed.19. The chip according to claim 17, wherein the chip is configured tomodify the respective bit allocation in a bit swapping procedure duringan online reconfiguration.
 20. The chip according to claim 17, whereinone or more of the groups includes tones within at least two differentrespective bands.
 21. The chip according to claim 17, wherein a singleband of the bi directional data communication is between a single userand a central office.